In conventional telephone networks (known colloquially as “POTS”—plain old telephone system) the end-to-end connection between a telephone exchange and customer premises is made by a branched network of wires along which an electric current can be transmitted, the current being modulated to provide analogue voice signals, and also digital pulses as used for dialling numbers, activating bells or other alerts (“ringing current”) and, more recently, for data transmission such as facsimile transmissions or computer connections to the “Internet”. The existence of an end-to-end electrical connection in conventional systems allows simple customer equipment (specifically telephone handsets) to be powered at low voltage from a power supply at the exchange. This was a particular advantage in the early development of telecommunications networks, when mains power supplies were less widespread than they have since become. It remains a useful feature that the telephone equipment is powered independently of the mains power supply to the customer premises, as in the event of failure of that supply, a user can still use the telephone to report the fact to the power supply company. Indeed, the user instructions for cordless telephones (which require a mains supply for the base station) strongly advise users to also maintain a fixed (wired-in) telephone on the premises for use in such eventualities.
The presence of an end-to-end wired network has also allowed intermediate elements (herein referred to as “nodes”) in the branched network to use electrical power delivered over the wires from elsewhere in the network. Many of these points are either in public locations (the familiar roadside “cabinets”, and the “distribution points” which are typically positioned at the top of wooden poles from where the “final drop” wire is connected to the customer premises). These nodes are unattended and often either in remote locations, where it may be difficult to arrange a mains power supply, or in public locations where the presence of a mains power supply could constitute a hazard to some members of the public, and a temptation to others. Hitherto, such nodes have had relatively modest power requirements, and provision of a low-voltage power supply from the exchange has been sufficient to avoid the need for a mains power supply to be provided to these nodes.
For the avoidance of doubt, the term “node” in this specification refers to elements within the network, as distinct from the “network termination point” (also known as the “network interface device” NID) located on a customer's premises which provides the interface between the network operator's equipment and that of the customer.
Modern developments in telephony have made this simple pattern less practical in recent times. In particular, the use of electromagnetic media (e.g. microwave or, more commonly, optical fiber) is already well-established in the trunk network (between exchanges) and is now becoming established in the local distribution network. Such arrangements are sometimes known as “fiber to the curb”, only the “final drop” from distribution point to customer premises remaining as conventional copper wire. The use of such connections allows much greater capacity. It is also cheaper as copper is relatively expensive compared with optical fiber (or indeed microwaves). However, electrical power cannot be provided to the network nodes over such media.
A typical arrangement is shown in FIG. 1. The exchange or central office 1 is connected by way of one or more intermediate nodes, known as cabinets 2 and distribution points 3, to the customer premises network termination equipment (NTE) 4. As shown in FIG. 1, the NTEs are connected to the distribution point 3, but longer connections where the termination is a long distance from any other termination may be connected directly to a cabinet 4 without an intermediate DP 3. Connected to the NTE 4 by internal connections 11, 13 are one or more standard telephones 5 and a “hub” (home gateway) 6 serving a home network 12 (wired or wireless) to which is connected one or more computer terminals 7, 8.
As shown in FIG. 1, the “final drop” 10 between the distribution point 3 and the customer premises NTE 4 is made by copper connection, but the connections 9 between the central office 1 and the cabinet 2, and between the cabinet 2 and the distribution point 3, are made by fiber optic connection.
Such arrangements break the hitherto continuous copper connection between the local exchange (central office 1) and the customer premises (NTE 4), and more specifically between the local exchange (central office 1) and the intermediate nodes (cabinet 2, distribution point 3). At the same time, the intermediate nodes 2, 3 now have greater power requirements than hitherto, in particular because transmission over the optical or microwave links itself requires power to generate the optical or microwave carrier.
However, most modern customer premises telecommunications equipment, such as computers, require much more power than can be supplied over the low-voltage telecommunications network 9, 10 and therefore have their own mains supply. It has been proposed for electrical power to be fed into the telecommunications network by way of the customer premises NTE 4 from a power feed 14, and through the “final drop” connection 10, (which remains wired) to the distribution point 3, as shown in FIG. 1. (To encourage the use of suitable customer premises equipment, the use of such equipment might be expected to attract a discount from the telecommunications provider). As each distribution point feeds a number of customer premises, the power required of each customer would be relatively small.
A problem with such an arrangement is that the network provider is reliant on the customers' own electrical power supplies to power the service. In the absence of a power supply from the exchange 1, even the standard plain old telephone services, or “POTS” have to be powered by the mains power supply 14 of the customer premises NTE 4. In the event of failure of that supply, the customer would be deprived of telephone service. This is considered unacceptable, as it would prevent vulnerable users making calls in an emergency—in particular, calls to report the power failure itself.